Particle Physics Booklet - Particle Data Group - Lawrence Berkeley ...

218 22. Dark matter
22. DARK MATTER
Revised September 2009 by M. Drees (Bonn University) and G. Gerbier
(Saclay, CEA).
22.1. Theory
22.1.1. Evidence for Dark Matter : The existence of Dark (i.e.,
non-luminous and non-absorbing) Matter (DM) is by now well established.
An important example is the measurement of galactic rotation curves. The
rotational velocity v of an object on a stable Keplerian orbit with radius
r around a galaxy scales like v(r) ∝ � M(r)/r, whereM(r) isthemass
inside the orbit. If r lies outside the visible part of the galaxy and mass
tracks light, one would expect v(r) ∝ 1/ √ r. Instead, in most galaxies one
finds that v becomes approximately constant out to the largest values of r
where the rotation curve can be measured. This implies the existence of
a dark halo, withmassdensityρ(r) ∝ 1/r2 , i.e., M(r) ∝ r, andalower
bound on the DM mass density, Ω >
DM ∼ 0.1.
The observation of clusters of galaxies tends to give somewhat larger
values, ΩDM � 0.2. These observations include measurements of the
peculiar velocities of galaxies in the cluster, which are a measure of their
potential energy if the cluster is virialized; measurements of the X-ray
temperature of hot gas in the cluster, which again correlates with the
gravitational potential felt by the gas; and—most directly—studies of
(weak) gravitational lensing of background galaxies on the cluster.
The currently most accurate, if somewhat indirect, determination of
ΩDM comes from global fits of cosmological parameters to a variety of
observations; see the Section on Cosmological Parameters for details. For
example, using measurements of the anisotropy of the cosmic microwave
background (CMB) and of the spatial distribution of galaxies, Ref. 3 finds
a density of cold, non–baryonic matter
Ωnbmh 2 =0.110 ± 0.006 , (22.1)
where h is the Hubble constant in units of 100 km/(s·Mpc). Some part of
the baryonic matter density [3],
Ωbh 2 =0.0227 ± 0.0006 , (22.2)
may well contribute to (baryonic) DM, e.g., MACHOs [4] or cold molecular
gas clouds [5].
The most recent estimate of the DM density in the “neighborhood” of
our solar system is 0.3 GeV cm−3. 22.1.2. Candidates for Dark Matter : Candidates for non-baryonic
DM in Eq. (22.1) must satisfy several conditions: they must be stable
on cosmological time scales (otherwise they would have decayed by now),
they must interact very weakly with electromagnetic radiation (otherwise
they wouldn’t qualify as dark matter), and they must have the right relic
density. Candidates include primordial black holes, axions, and weakly
interacting massive particles (WIMPs).
The existence of axions [9] was first postulated to solve the strong
CP problem of QCD; they also occur naturally in superstring theories.
They are pseudo Nambu-Goldstone bosons associated with the (mostly)
spontaneous breaking of a new global “Peccei-Quinn” (PQ) U(1) symmetry
at scale fa; see the Section on Axions in this Review for further details.
Although very light, axions would constitute cold DM, since they were